Presently, atmospheric density is measured by sending radiation through the atmosphere and measuring the intensity of the radiation returned. The returning radiation consists of radiation scattered by particles and molecules in the atmosphere. The intensity of the back-scattered radiation indicates the quantity of particles and/or molecules in a portion of the atmosphere and thus represents density.
One such measurement system is a lidar detector system. A lidar system consists of an internal light source which emits a high-energy optical pulse of very short duration and a receiver which optically collects and detects the light. In an atmospheric density measurement application, the light source is a laser and the receiver includes a telescope to receive and focus the backscattered optical energy. By measuring the intensity versus time relative to laser pulsing, a profile of the backscattering medium as a function of slant range from the lidar is obtained. Many lidars use a Nd:YAG laser transmitter having a radiation output of 1064 nm. Currently, there is no detector capable of making high-sensitivity measurements in the 1000 nm spectral region.
The lidar detector previously used for the detection of 1064 nm radiation utilized the Varian Model VPM-159 photomultiplier. These photomultipliers used a gallium arsenide-phosphide photocathode which was sensitive to radiation out to 1100 nm. It had a typical quantum efficiency of 3 percent at 1064 nm. When cooled to 77.degree. K., the tube had a dark count (counts created by noise of the detector itself) of typically 100 photoelectrons/sec. This photomultiplier was capable of counting single photoelectrons.
The VPM-159 had two characteristics which limited its usefulness. The photocathode was not a transmission type, as is standard on conventional photomultipliers, but was a small opaque cathode (0.220 by 0.250 in.) located behind the entrance window. The incident radiation was limited to an f/2.1 input cone without obscuration.
The second disadvantage of using this detector was that once manufactured it had to be continuously stored at a cold temperature of &lt;-20.degree. C., or else sensitivity degradation of the photocathode itself would occur. This introduced additional engineering complexity when the tube was installed in systems.
The two detector types currently used for lidar 1064 nm measurements are the avalanche silicon diode and the S-1 photomultiplier. The avalanche silicon diode is a high-resistivity photodiode designed to operate in the reverse voltage avalanche region just below the junction breakdown voltage. This results in a photocurrent gain proportional to the reverse voltage.
A major disadvantage of the avalanche diode is that the diode generates a relatively large amount of background noise. The TIED 69, a typical avalanche diode, has a gain of 600 with a 165 volt reverse bias. At 1064 nm and a range bin (sample unit denoting distance surveyed) of 150 m over 1 microsecond, the TIED 69 has an equivalent rms noise of approximately 760 counts per range bin. This noise is present in each one microsecond interval of diode readout.
A second potential detector for use at 1064 nm is a photomultiplier with an S-1 spectral response. The S-1 photocathode is relatively noisy at ambient temperature and must be cooled to reduce the noise. The large size of a photomutiplier introduces the requirement for a large cooling chamber to maintain the tube temperature at -100.degree. C. In addition, the quantum efficiency of the S-1 cathode is very low, typically 0.05 percent. Thus the S-1 photomultiplier also has inherent disadvantages for use as a 1064 nm lidar detector.
These conventional techniques cannot provide both high quantum efficiency and low intrinsic noise. Nor can they accumulate photon counts over a time interval without introducing noise into the count data.